Make the extension docs a top level entry in the landing page

Currently any documentation about extensions that Keystone offers
is lacking at best. This patch moves the documentation out of the
busy configuration section, and to index/landing page.

It also provides a brief summary for each extension, and a logical
point of entry for additional information.

Change-Id: Ide8ac9b277e0fc9cab9c01073bb72fe3bc369aed
Merge "Make the extension docs a top level entry in the landing page"